1. Field of the Invention
The present invention relates to digital radio, and in particular, though not exclusively to digital cellular radio.
2. Related Art
The form of modulation used in digital cellular radio systems depends on the radio environment in which a radio system is being used.
Binary modulation is usually preferred in wide area cellular radio systems as the cells tend to be large, forcing the radio communications to occur in the presence of low channel Signal-to-Noise Ratios (SNRs) and low signal-to-interference ratios. An advantage of binary modulation is that class-C amplification can be used, the high efficiency of which prolongs the life of battery portables compared to when linear amplification is used. As digital signal processing is essentially performed with bits, binary modulation is readily compatible with speech coding, channel coding and equalization.
Progressively, higher bit rates are being required of cellular radio systems. The transmitted bit rate can be increased by using multi-level modulation, where each symbol conveys more than one bit. Although the channel bandwidth need not be increased, many of the advantages associated with binary modulation cease. In particular, inefficient linear amplification is required, and the receiver must operate at high SNR levels. However, these difficulties become less significant if microcells or picocells are used. For these tiny cells, ranging from a kilometer along a motorway, to a 200 m section of a street to a single office, the radiated power may be below 10 mW and often of the order of a few microwatts. It is possible to operate in relatively high SNR environments, say in excell of 25 dB, and the inefficient linear amplifiers consume little battery power.
As many potential high bit rate communications may occur in office buildings, the capacity of mobile systems in these buildings must be very high. The high bit rates required to accommodate video, audio computer data and other traffic may necessitate the symbol rate of the multilevel systems being so high that the mobile radio channels exhibit dispersion. Performing equalization of multilevel signals e.g. multi-level Quadrature Amplitude Modulation (QAM), that are subjected to dispersion and fading is not an easy task.
A channel can be said to behave as a narrowband radio channel if the duration of the symbol transmitted is significantly greater than the delay spread of the channel. As a consequence, the received multipath signal is subjected to flat frequency facing. When the multipath signal has a dominant path Rician fading occurs. The worst case fading experienced in a narrowband channel is Rayleigh fading when very deep fades can occur, causing burst errors, even when the average SNR is high.
When the size of the cell is decreased the delay spread decreases, allowing an increase in the transmitted symbol rate whilst still maintaining flat fading conditions, i.e., avoiding the channel becoming dispersive resulting in Inter-Symbol Interference (ISI). As the cell size decreases to microcellular proportions, e.g., office microcells, the transmitted symbol rate may increase well above a mega-symbol/s.
However, even with very small microcells a point is reached when the delay spread becomes comparable with the symbol duration and ISI results. ISI can be combatted using symbol interleaving and channel coding. When the ISI becomes severe it is necessary to mitigate its effects by using channel equalization.
Many different types of equalizer systems are known. Almost all of them are designed to operate with binary modulation, rather than multi-level QAM. The major types of equalizers are linear (LE), decision feedback (DFE) and Viterbi. Usually the preferred equalizer is the Viterbi equalizer due to its theoretically optimal performance given perfect channel sounding, but unfortunately it is too complex for QAM as more than 2.sup.12 states are required.
Current QAM systems operate over fixed links, such as telephone wire circuits or point-to-point radio links, where the frequency selectivity is constant or changing very slowly compared to mobile channels. Generally a linear equalizer is adequate to overcome any ISI. The equalizer is normally made adaptive so that it automatically adjusts its tap coefficients as the channel dispersion changes. Occasionally DFEs are used which can also be adaptive.